Electrowetting is a fluidic phenomenon that enables changing of the configuration of a contained fluid system in response to an applied voltage. Examples of electrowetting optics may use two immiscible fluids having different properties. For example, the two fluids have different indices of refraction. One fluid may be conductive. The other fluid, typically the fluid adjacent to a hydrophobic surface, may be non-conductive, such as an oil. The conductive fluid may be a transparent liquid, but the other fluid may be reflective, transparent, or transmissive with a color tint. Where both liquids are transparent or transmissive, the non-conductive fluid typically exhibits a higher index of refraction than the conductive fluid. An electrowetting device may be controlled by changing the applied electric field, which in turn, applies a force to the conductive liquid to change the shape of the fluid interface surface between the two liquids and thus the refraction of the light passing through the interface surface. If the interface surface is reflective (e.g. due to reflectivity of one of the liquids or inclusion of a reflector at the fluid interface), changing the applied electric field changes the shape of the reflective interface surface and thus the steering angle of the light reflected at the interface surface. Depending on the application for the electrowetting optic, the light may enter the fluid system to pass first through either one or the other of the two liquids.
As outlined above, an electrowetting optic often includes a layer or the like formed on a hydrophobic material. A hydrophobic material is a substance that is not attracted to a polar liquid such as water. A hydrophobic material or layer thus may appear to repel water. In the absence of another force, a drop of water on a surface of a hydrophobic material exhibits a high contact angle with respect to the surface. Examples of a hydrophobic material for electrowetting applications often also form an electrically insulating dielectric. In such an optic, the hydrophobic dielectric layer often extends across one or more electrode surfaces within the optic that otherwise would be exposed to at least one of the liquids. Even small failures of the hydrophobic dielectric layer may compromise operation of the electrowetting optic including a potentially catastrophic failure, such as rupturing of the electrowetting optic from the gas build up due to hydrolysis.
FIG. 1A illustrates a simplified electrowetting cell 100 as is known in the electrowetting arts. The simplified electrowetting cell 100 is an enclosed capsule (e.g., in the shape of a six-sided, hollow cube) that contains immiscible fluids 103 and 105. The simplified electrowetting cell 100 is shown in a non-powered state in which the AC source 110 is not outputting any voltage to the electrodes 120A and 120B.
In the particular example of FIG. 1A, the electrowetting cell 100 includes a first fluid 103 and a second fluid 105 that are immiscible, and the first fluid 103 is conductive. The first fluid 103 may be an aqueous or water solution, and the second fluid 105 may be an oil, such as silicone oil. The electrowetting cell 100 also includes a hydrophobic dielectric layer 130 within the interior space of the electrowetting cell 100. Within the interior space of the electrowetting cell 100 are electrodes 120A and 120B coupled to a voltage source AC 110 that controls operation of the electrowetting cell 100. Adjacent to one of the electrodes, in this case electrode 120B, is the dielectric layer 130 of the enclosed electrowetting cell 100.
The liquid 103 responds to an electrical force created by a voltage applied by the AC power supply 110, to shape the liquid 105 in the interior of the electrowetting device 110. For example, the curved meniscus of the liquid 105 may cause input light to either focus or diverge depending upon the direction of the input light into the electrowetting cell 100. As is known in the electrowetting optic arts, different liquid surface shapes including waveforms, such as a sawtooth waveform, may be formed within an appropriately configured electrowetting device.
The following discussion proceeds to the illustration of FIG. 1B illustrates a failure of the dielectric layer 130. In the electrowetting cell 100 of FIG. 1B, the electrode 120B associated with the dielectric layer 130 is an anode electrode, and the electrode 120A opposite the dielectric layer 130 is a cathode. When the AC voltage is applied by the AC source 110 to the electrowetting cell 100, the shape of the meniscus between the water solution 103 and oil 105 may change from concave to convex, and the positively-charged hydrogen ions (H+) and the negatively-charged oxygen ions (O2−) generated within the aqueous or water solution 103 due to hydrolysis are distributed throughout the volume of the water solution 103. As illustrated, the movement 150 of the positively-charged hydrogen ions and the movement 160 of the negatively-charged oxygen ions within the water solution 103 depicts the distribution of the respectively charged ions throughout the water solution's volume. In addition, the electrical current 140 within the conductive water solution 103 is also shown traversing back and forth between electrodes 120A and 120B according to the applied AC voltage.
The electrowetting cell 100 of FIG. 1B includes the pair of electrodes 120A and 120B, and a dielectric 130 with hydrophobic properties in which a puncture or hole 188 is present in the dielectric 130. The hole 188 in the dielectric 130may result from a breakdown of the dielectric 130 due to a non-uniformity, or introduction of impurities into the dielectric 130, when depositing the dielectric 130 on the electrode 120B. The oil 105, as an insulator, acts as a dielectric; therefore the effects of the hole 188may be minimal in an operating state in which the oil covers the hole 188. However, in some states, when the electrowetting cell 100 is operating normally, the meniscus between the liquids 103 and 105 may shift to expose the area of the hole 188 to the water 103. In such a state, due to electrolysis in that particular region, the area of the region may breakdown more rapidly than other areas, such as an area of the dielectric frequently covered by the oil 105 during normal operation. While a thicker coating of dielectric 130 may increase the durability of the dielectric 130, a higher voltage must be used to operate the electrowetting cell 100.
In the AC example of FIG. 1B, the ions tend to distribute fairly uniformly throughout the conductive water solution 103. As shown in FIG. 1C, however, when a DC voltage having a particular polarity is supplied from the DC source 111 to the electrowetting cell 100, the oxygen ions and the hydrogen ions are drawn, as a result, to the electrodes 120A or 120B coupled to the opposite pole (negative or positive) of the DC source 111. The electrode 120A is coupled to the negative terminal of the DC source 111, and acts as a cathode. The electrode 120B is coupled to the positive terminal of the DC source 111, and acts as an anode.
After some time, the negatively-charged oxygen ions (O2−) migrate toward the electrode 120B as shown by the negative ION movement 162, and the positively charged hydrogen ions (H+) migrate toward the electrode 120A as shown by the positive ION movement 152. The electrical current 142 flows from the electrode 120B (anode) in the direction of the electrode 120A (cathode). As the negatively-charged oxygen ions begin to build up around the electrode 120B, the electrode 120B begins to oxidize. The electrode 120B oxidizes as illustrated in FIG. 1D enough that a buildup of Aluminum Oxide (Al2O3) forms a dielectric plug 189 that fills the dielectric hole 188. In this way, driving the cell for some time with a DC voltage can provide some degree of repair of the puncture in the dielectric layer 130.
In addition to the above, other similarly passive electrowetting repair methods include simply driving operation of electrowetting cell with an AC signal with a DC offset. The DC offset in combination with the AC signal degrades the operation of the electrowetting cell under normal operating conditions. Exacerbating the situation is the fact that during a failure condition, the AC signal is not conducive to an electrowetting cell repair process.